1. Field of the Invention
The present invention relates to a battery exhaust gas plug filter having water repellency, gas permeability and strength applicable enough to the practical use.
2. Description of the Prior Art
A filter comprising a lead battery exhaust plug is required to have water repellency, gas permeability and strength.
When charging a lead battery, the electrolyte decomposes electrically to generate hydrogen and oxygen gases. If the filter does not have enough gas permeability, these gases reside in the lead battery, and causes explosion and accumulation of internal pressure. Thus the electrolyte like strong acid leaks out, and apparatuses around the lead battery are damaged by corrosion.
If the filter does not have water repellency, the electrolyte might leaks out even due to a light vibration. Moreover, the filter should have strength to resist vibrations and the like.
Various filters have been investigated to satisfy these requirements. Currently, the following filter is widely used: a filter made of an inorganic substance having good anti-electrolyte property and heat resistance, such as fused alumina, silicon carbide or silica porous substance, and treated with silicon resin water repellent agent.
The water repellency of an exhaust gas plug using the filter mentioned above deteriorates due to splashing electrolyte casued by the vibrated battery. As a result, the electrolyte has leaked through the exhaust gas plug.
A process, in which a porous substance made of fused alumina is treated with water repellent fluororesin dispersion to obtain a filter, has been proposed. However, the electrolyte leaks out through the exhaust gas plug even by a light vibration. It is believed that the leakage is caused by the following reasons: The fluororesin gets together locally, solidifies and blocks some of larger pores, and the inside of smaller pores are not covered with the fluororesin at all.
Further, a filter is proposed in Japanese Examined Patent Publication (KOKOKU) No. 8510/1968 in which a porous film containing fluororesin with water repellency is formed on end surfaces of a porous substance made of fused alumina. And another filter is proposed in Japanese Unexamined Patent Publication (KOKAI) No. 161656/1986 in which a gas permeation film comprising water repellent polytetrafluoroethylene or high density polyethylene covers the top surface, the side surface or the front surface of a filter, disposed in an exhaust gas plug, comprising a porous substance made of ceramic or synthetic resin.
However, since the films obtained by these processes do not have enough gas permeability, generated gases reside in the battery. And since the films are weakly bonded to the porous substance, the films are liable to come off the porous substance and break. Consequently, the electrolyte gets into the inside of the porous substance, the pores are blocked, and the explosion of the battery might occur.